1,837 research outputs found
Weak ferromagnetism of La_{1.99}Sr_{0.01}CuO_4 thin films: evidence for removal of corrugation in CuO_2 plane by epitaxial strain
The weak ferromagnetism of La_{1.99}Sr_{0.01}CuO_4 epitaxial thin films is
investigated using magnetoresistance measurement. While a steplike negative
magnetoresistance associated with the weak ferromagnetic transition is clearly
observed in the films grown on YAlO_3(001), it is notably suppressed in the
films grown on SrTiO_3(100) and
(LaAlO_3)_{0.3}(SrAl_{0.5}Ta_{0.5}O_3)_{0.7}(100), and almost disappears in
films grown on LaSrAlO_4(001). The strong suppression of the steplike
magnetoresistance provides evidence that the CuO_2 planes are much less
corrugated in thin films grown on tetragonal substrates, particularly on
LaSrAlO_4(001), than in bulk crystals.Comment: 6 pages, 6 figures, to be published in Physical Review
Structure of the exotic spin-flop states in BaCu2Si2O7
The unusual 2-stage spin flop transition in BaCu2Si2O7 is studied by
single-crystal neutron diffraction. The magnetic structures of the various
spin-flop phases are determined. The results appear to be inconsistent with the
previously proposed theoretical explanation of the 2-stage transition.Comment: 6 pages 5 figure
Magnetic anisotropy of BaCu2Si2O7: theory and antiferromagnetic resonance
Antiferromagnetic resonance (AFMR) of BaCu2Si2O7 and a microscopic theory of
the magnetic anisotropy of spin 1/2 chain compounds with folded CuO3 geometry
being in good agreement with the available data are presented. The AFMR studies
at 4.2 K show the existence of two gaps (40 and 76 GHz) at zero magnetic field
and of two spin re-orientation transitions for H||c. The microscopic origin of
the two gaps is shown to be Hund's rule coupling which leads to a "residual
anisotropy" beyond the compensation of the Dzyaloshinskii-Moriya term by the
symmetric anisotropy which would be valid without Hund's coupling.Comment: 4 pages, 3 figure
Doping evolution of the electronic specific heat coefficient in slightly-doped La2-xSrxCuO4 single crystals
Detailed doping dependence of the electronic specific heat coefficient gamma
is studied for La2-xSrxCuO4 (LSCO) single crystals in the slightly-doped
regime. We find that gamma systematically increases with doping, and
furthermore, even for the samples in the antiferromagnetic (AF) regime, gamma
already acquires finite value and grows with x. This suggests that finite
electronic density of states (DOS) is created in the AF regime where the
transport shows strong localization at low temperatures, and this means the
system is not a real insulator with a clear gap even though it still keeps long
range AF order.Comment: 4 pages, 4 figures, accepted for publication in Journal of Physics:
Conference Series (LT25 proceeding
Nature of Chemisorption on Titanium Carbide and Nitride
Extensive density-functional calculations are performed to understand atomic
chemisorption on the TiC(111) and TiN(111) surfaces, in particular the
calculated pyramid-shaped trends in the adsorption energies for second- and
third-period adatoms. Our previously proposed concerted-coupling model for
chemisorption on TiC(111) is tested against new results for adsorption on
TiN(111) and found to apply on this surface as well, thus reflecting both
similarities and differences in electronic structure between the two compounds.Comment: 7 pages, 4 figures, conference proceeding presented at IWSP-2005
(Polanica Zdoj, Poland, 2005), submitted to Surf. Sci. (2005
Hall effect in superconducting Fe(Se0.5Te0.5) thin films
The Hall effect is investigated for eight superconducting Fe(Se_0.5_Te_0.5_)
thin films grown on MgO and LaSrAlO_4_ substrates with different transition
temperatures (T_c_). The normal Hall coefficients (R_H_) have positive values
with magnitude of 1 - 1.5 x 10^-3^ cm^3^/C at room temperature for the all
samples. With decreasing temperature, we find two characteristic types of
behavior in R_H_(T) depending on T_c_. For thin films with lower T_c_
(typically T_c_ < 5 K), R_H_ start decreasing approximately below T = 250 K
toward a negative side, some of which shows sign reversal at T = 50 - 60 K, but
turns positive toward T = 0 K. On the other hand for the films with higher T_c_
(typically T_c_ > 9 K), R_ H_ leaves almost unchanged down to T = 100 K, and
then starts decreasing toward a negative side. Around the temperatures when
R_H_ changes its sign from positive to negative, obvious nonlinearity is
observed in the field-dependence of Hall resistance as to keep the low-field
R_H_ positive while the high-field R_H_ negative. Thus the electronic state
just above T_c_ is characterized by n_e_ (electron density) > n_h_ (hole
density) with keeping \mu_e_ < \mu_h_. These results suggest the dominance of
electron density to the hole density is an essential factor for the occurence
of superconductivity in Fe-chalcogenide superconductors.Comment: 11 pages, 4 figures, revised version for Physical Review B. accepted
for publication in Physical Review
Two-stage spin-flop transitions in S = 1/2 antiferromagnetic spin chain BaCu_2Si_2O_7
Two-stage spin-flop transitions are observed the in quasi-one-dimensional
antiferromagnet, BaCuSiO. A magnetic field applied along the
easy axis induces a spin-flop transition at 2.0 T followed by a second
transition at 4.9 T. The magnetic susceptibility indicates the presence of
Dzyaloshinskii-Moriya (DM) antisymmetric interactions between the intrachain
neighboring spins. We discuss a possible mechanism whereby the geometrical
competition between DM and interchain interactions, as discussed for the
two-dimensional antiferromagnet LaCuO, causes the two-stage
spin-flop transitions.Comment: 5 pages, 3 figures (included), accepted for publication in Phys. Rev.
Let
Transport properties and electronic states of the thermoelectric oxide NaCo_2O_4
Physical properties of the thermoelectric oxide NaCo_2O_4 are briefly
reviewed. The high thermoelectric properties of this material are attributed to
the substantially enhanced effective mass, which comes from the large entropy
of Co in the low spin state. The large entropy confined in the CoO_2
block causes a spin-density-wave transition at 22 K upon Cu substitution for
Co, which can be regarded as ``order from disorder''.Comment: 4 Pages, $ figures, Proceedings of The 2nd Hiroshima Workshop:
Transport and Thermal Properties of Advanced Material
Ferromagnetism and large negative magnetoresistance in Pb doped Bi-Sr-Co-O misfit-layer compound
Ferromagnetism and accompanying large negative magnetoresistance in
Pb-substituted Bi-Sr-Co-O misfit-layer compound are investigated in detail.
Recent structural analysis of (Bi,Pb)SrCoO, which has
been believed to be a Co analogue of
BiSrCaCuO, revealed that it has a more complex
structure including a CoO hexagonal layer [T. Yamamoto {\it et al.}, Jpn.
J. Appl. Phys. {\bf 39} (2000) L747]. Pb substitution for Bi not only
introduces holes into the conducting CoO layers but also creates a
certain amount of localized spins. Ferromagnetic transition appears at =
3.2 K with small spontaneous magnetization along the axis, and around the
transition temperature large and anisotropic negative magnetoresistance was
observed. This compound is the first example which shows ferromagnetic
long-range order in a two-dimensional metallic hexagnonal CoO layer.Comment: 8 pages including eps figures. To be published in J. Phys. Soc. Jp
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